1. Field of the Invention
The present invention relates to a radial foil bearing.
2. Description of Related Art
Conventionally, a radial bearing that is used by being fitted on a rotating shaft is known as a bearing for a high-speed rotating body. As this kind of radial bearing, a radial foil bearing is well known that is provided with a thin plate-shaped top foil that forms the bearing surface, a back foil that resiliently supports the top foil, and a cylindrical bearing housing that houses the top foil and the back foil. As the back foil of the radial foil bearing, a bump foil that consists of a thin plate molded in a corrugated shape is chiefly used.
In the case of this kind of radial foil bearing, in order to prevent the top foil and the bump foil from dropping out of the bearing housing, normally one end portion thereof (toe portion) is directly fixed to the bearing housing via spot welding or indirectly fixed via a spacer.
Also, in Patent Document 1, both ends of the top foil are locked and fixed by respectively being made to butt against stop walls of the internal wall of a housing.    [Patent Document 1] Japanese Unexamined Patent Application, First Publication No. 2006-57828    [Patent Document 2] Japanese Unexamined Patent Application, First Publication No. 2006-57652    [Patent Document 3] Japanese Unexamined Patent Application, First Publication No. 2004-270904    [Patent Document 4] Japanese Unexamined Patent Application, First Publication No. 2009-299748
However, when welding the top foil, it is highly likely that strain will be produced in the top foil due to the heat input. Also, while there is known one that performs a bending process on one end portion (toe portion) of the top foil to perform mechanical fixing as a substitute for welding, in that case, strain ends up being produced in the top foil by the bending process. Moreover, since both ends of the top foil are butted against the stop wall in Patent Document 1, a reaction force is applied from both end portions of the top foil toward the center portion, resulting in strain being produced.
However, the lubricating fluid film of the foil bearing that is formed between the rotating shaft and the top foil due to the rotation of the rotating shaft is extremely thin at around 10 micrometers. For this reason, even a little strain in the top foil affects the load capability and the dynamic characteristics (rigidity and attenuation) of the bearing, and so the as-designed performance is no longer obtained.
Also, in the case of a common top foil in which one end portion (toe portion) is fixed to the bearing housing by spot welding, the vicinity of both ends (the toe end side and free end side) enters a plane-like state that is hindered from adapting to the curved surface that constitutes the inner circumferential surface of the bearing housing. Thereby, a force that constricts the rotating shaft (local preload) is produced at the section close to the plane. As a result, problems arise such as the starting torque increasing, and the heat generation during operation of the rotating shaft becoming higher than the setting.
Also, since strain is produced in the top foil by the reaction force in Patent Document 1, the top foil ends up becoming a shape close to square that partially has planar portions due to the strain, instead of a shape that is close to perfect circle following the inner circumferential surface of the bearing housing. Thereby, due to the section near the planar portion making strong contact with the rotating shaft, a force that constricts the rotating shaft (local preload) is produced, leading to an increase in the starting torque, and the heat generation during operation of the rotating shaft 1 becoming higher than the setting.
In order to reduce this force that constricts the rotating shaft (local preload), methods are conceivable to eliminate the mountain of the bump foil (back foil) that supports both end regions of the top foil. However, when the mountain of the bump foil is eliminated, since the support rigidity of the rotating shaft at the sections where the mountain was eliminated falls drastically, the restraint is not effective when the rotating shaft tries to move toward that section due to an impact load or the like, and so there is an increase in the possibility of a rotating portion such as an impeller that is provided on the rotating shaft making contact with the static portion (housing portion).
Also, a method is also conceivable to reduce the bump foil of the aforementioned section by only one mountain so as to not excessively lower the support rigidity of the rotation shaft at that section. However, since the reduction amount is on the order of several tens of micrometers, its manufacture is extremely difficult.